A conventional pneumatic tire may have sipes in tread blocks so that the running performance of the tire improves on ice- or snow-covered roads due to the edges of the sipes (known as a studless tire). The studless tire may be used on vehicles which run on ice- or snow-covered roads.
Because groove width of a sipe is narrow, when the sipe opens, it is easy for stress to concentrate on the bottom portion of the sipe. It is therefore easy for cracks to form from the bottom portion of the sipe. In order to combat this drawback, an enlarged portion, having a circular cross-sectional configuration, at the bottom portion of a sipe may be formed. However, when two sipes are disposed so as to be adjacent to each other, if an interval between enlarged portions is too narrow, the rubber, which should be between the sipes, may be caught and remain between blades of a vulcanization mold. Further, the rigidity of the narrow area between the sipes may decrease and the amount of shearing deformation, when the pneumatic tire is subject to a front-to-back force during running, may increase. Thus, cracks may form at the bottom portion of the sipe.
Conventional blocks may have at least two sipes. When side force is applied to narrow areas due to cornering or the like, distortion due to shearing occurs, and cracks form in the bottoms of the sipes. If the cracks worsen, depending on the case, the narrow areas may break off.
However, when a side force occurs during running, such as that described above, and acts upon tires having a variety of configurations for blocks having sipes, the bottom portion of the narrow area may be deformed by shearing force in the transverse direction. Again, cracks are generated, and, depending on the case, the narrow area may break off.
When the above-described blocks are disposed on a pneumatic tire tread such that the narrow areas are arranged in a row along the transverse direction of the tire, areas having low rigidity are conventionally arranged so as to coincide along the transverse direction of the tire. When a side force is applied to the pneumatic tire, the deformation of the narrow areas may be large and cracks may form in the narrow areas and the narrow areas may break off.
Usually, concave portions for forming blocks are provided in a vulcanization mold for vulcanizing a block tire. After an unvulcanized green tire is placed in the vulcanization mold, the green tire is pressurized to a predetermined pressure and heated to a predetermined temperature. Due to this process, the rubber of the green tire is pressed tightly into the concave portions so that the outer contour is formed and vulcanization is effected.
When internal pressure is applied to the green tire and rubber of the green tire is forced into a concave portion of the mold, the concave portion is closed by the rubber and air and gas generated by the rubber stagnate so that the rubber cannot flow. In order to prevent such a situation from occurring, vent holes, which communicate with the outside air, may be provided in vicinities of the four corners of the concave portion so that the air within the cavities may escape and the rubber may flow more easily. When a pair of sipes is provided so that the block is divided into three areas, it may be necessary to form a pair of thin, plate-shaped blades, which extend from one wall surface of the mold to the opposing wall surface, within the concave portion. The concave portion may be divided into three areas by the pair of blades so that a small concave portion may be formed between the pair of blades. When the rubber of the green tire flows into the concave portion, there is no place within the small convex portion for the air to escape so that the small concave portion may be closed by the rubber. Therefore, the rubber does not flow to the bottom portion of the small concave portion, and bare areas exist in the block after vulcanization.
In order to eliminate this drawback, one conventional method forms a pneumatic tire by using a vulcanization mold in which through-holes are provided so that the small concave portion and end concave portions, which are adjacent to the small concave portion, communicate via the through-holes. However, this structure does not sufficiently allow the air inside the small concave portion to escape, and consequently does not sufficiently prevent the formation of bare areas.
A conventional pneumatic tire, as the tire becomes worn, may prevent the protruding of a narrow area interposed between sipes so that the on-ice running performance of the tire does not deteriorate. The conventional tire may have blocks with at least one pair of sipes and which may reliably and easily be removed from a mold for vulcanization. The tire may have blocks with at least one pair of sipes in which the generation of cracks at bottom portions of the sipes may be prevented. The tire may further have a block pattern capable of preventing the formation of cracks in a narrow area defined by a pair of sipes and the breaking off of portions of blocks. A mold for vulcanization of the conventional tire may prevent the formation of bare areas in a narrow area when the pneumatic tire is manufactured.
The conventional pneumatic tire may comprise a plurality of blocks, each having a narrow area interposed between one set of two sipes which extend substantially in a transverse direction of the tire and wide areas positioned respectively at both sides of the narrow area across the sipes. As the tire contacts the ground during rotation, due to the difference between the distance from the center of rotation of the tire to the tread surface of the narrow area and the distance from the center of rotation of the tire to the tread surface of the wide area, the narrow area may intermittently slidingly contact the road surface and not contact the road surface. Thus, the narrow area may wear more quickly than the wide areas due to the narrow areas slidingly contacting the road surface. Therefore, the narrow area does not protrude compared with the wide areas. The rigidity of the narrow area may not be as small as that of the wide area.
Another conventional pneumatic tire may comprise a plurality of blocks, each having a narrow area interposed between one set of two sipes which extend substantially in a transverse direction of the tire and wide areas positioned respectively at both sides of the narrow area across the sipes, expanded portions being formed respectively at bottom portions of the sipes. By providing the expanded portions at the bottom portions of the sipes, the stress applied to the bottom portions when the sipes open may be dispersed, and the formation of cracks from the bottom portions may be prevented. Therefore, when the pneumatic tire is removed from the vulcanization mold used to form the pneumatic tire, the narrow area is not caught and does not remain between blades of the vulcanization mold which are used to form the sipes.